1. Field of the Invention
The present invention relates to a method for controlling a crosslinking degree of each layer in a laminate film and a multi-layered crosslinked ethylenic resin film obtained by such a method.
The film is advantageously applicable to shrink packaging.
2. Prior Art Statement
A conventional method for producing multi-layered crosslinked ethylenic resin films is known by, for example, U.S. Pat. No. 3,022,543. Electron beam-irradiated, crosslinked films obtained by this method are excellent in strength, transparency and heat shrinkable properties but involve drawbacks that due to an insoluble gel caused by the crosslinking, heat sealing characteristics are poor and packaging operation cannot be performed efficiently.
In order to improve the heat sealing characteristics, some methods for forming a film in multi-layers and controlling its crosslinking degree have been proposed. There are, for example, (1) a method in which an antioxidant such as di-t-butylcresol (BHT) or the like as a crosslinking-controlling agent is used in large quantities (more than 0.5 wt %) as shown in Japanese Patent Application KOKAI No. 50-12167, (2) a method in which a polyfunctional monomer such as diallyl maleate or the like is used as a crosslinking accelerator as shown in Japanese Patent Application KOKAI No. 55-95568, (3) a method in which a condensed polycyclic aromatic compound such as benzylnaphthalene or the like is used as a crosslinking-controlling agent as shown in Japanese Patent Application KOKAI No. 58-18228; and the like. In short, these methods are all intended to obtain an inner layer for enhancing stretching stability and imparting mechanical properties as a crosslinking film and a surface layer in a poorly crosslinked or non-crosslinked state under irradiation of electron beams in a dosage on the same level. According to the respective examples, it appears that the intended effect of controlling the crosslinking degree would apparently be exhibited.
On the other hand, Japanese Patent Application KOKAI No. 51-133368 discloses that polybutene, polyisobutylene, hydrogenated polybutene, rosins or polyterpenes are added as ordinary additives to a single layer electron beam-crosslinked film. However, this method lacks any technical idea of controlling the crosslinking degree using these additives.
However, the crosslinking-controlling agents used in (1) and (3) described above are low molecular weight substances so that these methods involve defects that (a) the substances evaporate off at a die outlet upon extrusion of a molten blend with resin at a high temperature to reduce its concentration and their crosslinking controlling effect cannot be sufficiently exhibited; (b) the evaporated additives adhere to a die outlet to deteriorate thickness deviation of the film; (c) when used in large quantities, extrusion stability is injured. The crosslinking accelerator in (2) described above has defects that the accelerator tends to be thermally crosslinked and bring about fish eye, during the course of blending with resin, melting the blend and extruding the molten blend at a high temperature.
Further films containing the crosslinking-controlling agents in (1), (2) and (3) encounter a problem that their application is limited, when the films are intended to use for food packaging. On the other hand, packaging films are used for various applications and often brought into contact with foods, once the films are released in the market. Therefore, naturally in the case for food packaging but also in general packaging films, close attention should be paid to hygienic safety of films.
In the light of the foregoing, there still remains the problems that any useful method for controlling a crosslinking degree, without reducing productivity, has not been developed under the actual situation and that food packaging films using such a method have not yet been commercially available. Furthermore in the field of shrink packaging using crosslinked films, additional problems encounter that packaging steps are complicated and troublesome, since films excellent in heat sealing characteristics are not present as described above.